In recent years, the objective of electrophotographic apparatuses has been changing from office-use to personal-use, and there have been increasing demands for techniques for achieving small-size and maintenance-free apparatuses. For this reason, conditions, such as a superior maintenance property for recycling a waste toner and reduced ozone generation, need to be satisfied.
The following description will discuss a printing process carried out by a copying machine and a printer of an electrophotographic system. First, an image-bearing member (hereinafter, referred to as a photosensitive member) is charged so as to form an image. As to the charging method for evenly charging a surface of a photosensitive member, a corona charger may be used as has been conventionally used, or in recent years, a contact-type charging method in which a conductive roller is directly pressed onto a photosensitive member has been adopted in an attempt to cut generation of ozone. In the case of a copying machine, after a photosensitive member has been charged, light is directed to an original material to be copied and the reflected light is directed to a photosensitive member through a lens system. Alternatively, in the case of a printer, an image signal is sent to a light-emitting diode or a laser diode serving as an exposing light source so that a latent image is formed on a photosensitive member based on ON-OFF operations of light. When the latent image (resulting from high and low portions of the surface potential) has been formed, the latent image on a photosensitive member is converted into a visible image by toner that is preliminarily charged color powder (having a diameter of approximately 5 μm to 15 μm). The toner is allowed to adhere to a surface of a photosensitive member in accordance with the high and low portions of the surface electric potential of a photosensitive member, and electrically transferred onto a sheet of transfer paper. In other words, the toner, which has been preliminarily charged positively or negatively, is electrically absorbed by applying a charge having an opposite polarity to the toner polarity from behind the transfer paper. As to a transferring method, the conventional method using a corona charger may be used, or a recently-developed contact-type transfer method in which a conductive roller is directly pressed onto a photosensitive member has been put to practical use in an attempt to cut generation of ozone. At the time of the transferring process, all the toner on a photosensitive member is not necessarily transferred onto a sheet of transfer paper, and one portion thereof remains on a photosensitive member. This residual toner is scraped by a cleaning blade, etc., in a cleaning section to form a waste toner. Then, the toner that has been transferred onto the transfer paper is fixed onto a sheet of paper by heat and pressure applied in a fixing process.
As to the fixing method, there are proposed a pressure fixing system in which a sheet of paper is allowed to pass through not less than two metal rolls, an oven fixing system in which the paper is allowed to pass through an atmosphere heated by an electric heater and a heat roll fixing system in which the paper is allowed to pass through heated rollers. In the case of the heat roll fixing system, a preferable thermal efficiency is obtained at the time when the toner image is fused onto the sheet of transfer paper because the surface of the heating roller and the toner surface on the sheet of transfer paper are made in press-contact with each other, thereby making it possible to carry out the fixing process quickly. However, in the case of the heat roll fixing system, the toner in a heated and melted state is made in press-contact with the surface of the heating roller, with the result that one portion of the toner tends to adhere to the roller surface to again adhere to the sheet of transfer paper, resulting in a stained image, which phenomenon is referred to as an offset phenomenon. As to a method for preventing the offset phenomenon, a method has been proposed in which the surface of the heating roller is formed by fluorine resin or silicone rubber that has a heat resisting property and a superior mold-releasing property to toner, and an anti-offset liquid such as silicone oil is supplied onto the surface so as to coat the roller surface with a thin-film of the liquid. In this method, however, when the liquid such as silicone oil is heated, an offensive odor is generated, and additional devices are required so as to supply the liquid, making the mechanism of the copying machine complex. Moreover, in order to prevent the offset in a stable manner, it is necessary to control the supply of the liquid with high precision, and this causes high costs of the copying machine. Therefore, there have been demands for a toner which provides a superior fixed image and is free from an offset, without the necessity of supplying such a liquid.
As has been generally known, an electrostatic charge developing toner, used for an electrophotographic method, is generally composed of a resin component, a coloring component formed by a pigment or dye, a plasticizer, a charge control agent and an additive component such as a mold-releasing agent to be added, if necessary. As to the resin component, a natural or synthetic resin is used alone or in combination as the resin component.
Then, the additive agents are preliminarily mixed at an appropriate ratio, and heated and kneaded in a thermally molten state, and this is finely ground through an air-flow collision plate system, and then finely classified to form a toner base material. Then, an external additive agent is externally added to this toner base material, thereby forming a toner.
In mono-component developing system, only the toner is used, and in the case of a two-component developing agent, the toner and a carrier composed of magnetic particles are mixed.
In a color copying machine, a photosensitive member is charged by a corona discharge using a static charger, and latent images of respective colors are applied to a photosensitive member as light signals to form electrostatic latent images, and this is developed by, for example, a yellow toner serving as a first color, so as to visualize the latent image. Thereafter, a transfer member, which has been charged to a polarity opposite to the charge of the yellow toner, is made in contact with a photosensitive member so that the yellow toner image, formed on a photosensitive member, is transferred thereon. After residual toner from the transferring process has been cleaned therefrom, a photosensitive member is subjected to a static charge eliminating process, thereby completing the developing and transferring processes of the first color toner.
Thereafter, the same processes as the yellow toner are repeated as to toners of magenta and cyan so that the toner images of the respective colors are superimposed on a transfer member to form a color image. These superimposed toner images are transferred onto a sheet of transfer paper that has been charged to a polarity opposite to the toner, and then fixed, thereby completing the copying process.
As to the color-image forming method, generally-used systems are: a transfer drum system in which toner images of the respective colors are successively formed on a single photosensitive member, and a transfer member wrapped on the transferring drum is rotated and allowed to face a photosensitive member repeatedly so as to successively superimpose the toner images of respective colors thereon, and a continuous superimposing system in which a plurality of image-forming units are placed side by side, and a transfer member, transported by a belt, is allowed to pass through the respective image-forming units so as to successively transfer toner images of respective colors thereon, thereby superposing the color images.
Here, for example, Japanese Patent Kokai Publication No. 250970/1989 (H1-250970) discloses a color image-forming apparatus using a continuous transferring system. In this conventional apparatus, four image-forming stations, each containing a photosensitive member, an optical scanning means, etc. for forming an image having each of four colors, are placed side by side, and a sheet of paper, transported by a belt, is allowed to pass below the respective photosensitive members so that color toner images are superimposed thereon.
Moreover, based on another method for forming a color image by superimposing toner images of different colors on a transfer member, Japanese Patent Kokai Publication No. 212867/1990 (H2-212867) has disclosed a method in which toner images of respective colors, which have been successively formed on a photosensitive member, are once superimposed on an intermediate transfer member, and the toner images on this intermediate transfer member are lastly transferred on a sheet of transfer paper in one batch.
Here, from the viewpoint of the recent earth environmental protection, there have been demands for reduction in generation of ozone, recycling a waste toner that has been disposed without being recycled, so as to regulate limitless dumping of industrial wastes, and a low-temperature fixing method for reducing the power consumption of the fixing process. The toner materials have also been improved so as to meet the roller transfer method that is less likely to produce generation of ozone, a waste-toner recycling system and a low-temperature fixing process. Thus, from the viewpoint of the environmental protection, it has been an important subject to develop a high performance toner that satisfies not only one of these objectives, but all these objectives simultaneously.
Moreover, in copying machines, printers and facsimiles, different kinds of toners are used for respective model types having different processing speeds. For example, in a low-speed machine, a binding resin material having high viscoelasticity and high softening point is used so as to improve anti-offset property. In a high speed machine which has difficulty in obtaining an amount of heat required for the fixing process, another binding resin having different property such as reduced softening point is used so as to increase fixing property. The processing speed relates to a copying process capability per unit of time of a machine, and represents a peripheral velocity of a photosensitive member. Depending on the peripheral velocity of a photosensitive member, the transporting velocity of sheets of transfer paper is determined. If these different toners are unified and commonly used, it is possible to increase the production efficiency, and also to reduce the costs of toner.
In a fixing process, fixing strength represented by adhesive strength of a toner to paper and anti-offset property for preventing adhesion to a heat roller form controlling factors.
A toner is melted and allowed to permeate into fibers of paper by heat or pressure from the fixing roller so that fixing strength is obtained. Conventionally, in order to improve fixing property, the binding resin is improved and a mold-releasing agent is added so that the fixing strength for sticking to paper is improved, and it is possible to prevent the offset phenomenon in which toner adheres to the fixing roller.
Japanese Patent Kokai Publication No. 148067/1984 (S59-148067) has disclosed a toner which uses as a resin an unsaturated ethylene polymer having a low molecular weight portion and a high molecular weight portion in which the peak value of the low molecular weight portion and the ratio Mw/Mn are limited and which also contains polyolefin whose softening point is specified. This application suggests that this composition ensures proper fixing property and anti-offset property. Further, Japanese Patent Kokai Publication No. 158340/1981 (S56-158340) has disclosed a toner mainly composed of a resin constituted by a specific low molecular weight polymer component and high molecular weight polymer component. The objective of this disclosure is to ensure a proper fixing property by using a low molecular weight component, while ensuring anti-offset property by using a high molecular weight component. Moreover, Japanese Patent Kokai Publication No. 223155/1983 (S58-223155) has disclosed a toner which contains a resin made from an unsaturated ethylene polymer having maximum values in respective molecular weight ranges of 1,000 to 10,000 and 200,000 to 1000,000 and a ratio of Mw/Mn of 10 to 40, and polyolefin having a specific softening point. The objective of this composition is to ensure a proper fixing property by using a low molecular weight component, while ensuring a proper anti-offset property by using a high molecular weight component and the polyolefin.
However, in the case when, in order to increase fixing strength in a high-speed apparatus, melt viscosity of a binding resin is reduced or a resin having a lowered molecular weight is used, the toner tends to have a so-called spent phenomenon in which the toner sets to the carrier, when used for a long time in the case of a two-component developing process. In the case of a mono-component developing process, a toner tends to set to a doctor blade and a developing sleeve, resulting in reduction in resistance to stress in the toner. Moreover, when this is applied to a low-speed apparatus, an offset in which a toner adheres to a heat roller, tends to occur at the time of fixing. Furthermore, blocking in which toner particles are melted to adhere to each other, tends to occur after long-term storage.
In these compositions in which a high molecular weight component and a low molecular weight component are blended, although it is possible to satisfy both the fixing strength and anti-offset property based on process speeds of narrow range, it is difficult to satisfy these based on process speeds of wide range. In order to deal with process speeds within wide range, it is possible to obtain certain effect by using a higher high molecular weight component and a lower low molecular weight component. However, in the case of a high-speed apparatus, fixing strength may be improved by increasing a low molecular weight component, but results in degradation in anti-offset property. In the case of a low-speed apparatus, anti-offset property is improved by increasing a high molecular weight component, but causes reduction in toner grindability, results in reduction of productivity.
For this reason, to a composition in which a high molecular weight component and a low molecular weight component are blended or copolymerized is added a mold-releasing agent having low melting point, such as polyethylene or polypropylene wax, in order to improve mold-releasing property from a heat roller at the time of fixing and to enhance anti-offset property.
However, these mold releasing agents hardly disperse in a binder resin, and toners having reversed polarity tends to appear due to insufficient dispersion, results in fog at a non-image portion. Moreover, an image loss, which looks as if it were rubbed by a brush, tends to occur at the rear end of a solid black image portion, resulting in degradation in image quality. Another problem is filming contamination that tends to occur in a carrier, a photosensitive member and a developing sleeve.
In a method for heating and kneading an internal additive agent such as a mold-releasing agent and dye so as to disperse it in a binder resin through a thermal melting process, devices, such as a roll mill, a kneader and an extruder, have been conventionally used in kneading process that forms an important position in the toner manufacturing process.
This extruder with twin screws is a twin-screw extruder with shallow grooves of a meshed type in which kneading screws are rotated at high speed, and as to the kneading screws, a selection is made between a same-direction rotary mode of a completely meshed type and a different direction rotary mode of a partially meshed type depending on materials. The cylinder and the kneading screws employ a divided segment system. As to a plurality of divided segments, a heating cylinder is installed in each segment so as to set a specific kneading temperature, and cooling water is allowed to flow through it. The kneading screw which passes through the cylinder, is constituted by a feeding portion that mainly has a feeding function for feeding a kneading matter forward with melting it by heating, and a kneading portion that mainly has a kneading function. The feeding portion has spiral shaped structure and has comparatively low kneading force exerted by shearing action, while the kneading portion carries out a kneading process by strong shearing force.
In order to increase dispersing property in these kneading processes, Japanese Patent Kokai Publication No. 194878/1994 (H6-194878) discloses that temperature of a cylinder in a kneader is set within 20 K based on lowest temperature of a kneaded matter extruded from the kneader. This application suggests that this arrangement allows the resin to be sufficiently melted while a kneaded matter of toner materials is transported through the cylinder during the kneading process, that no reduction in viscosity occurs due to an unmelted matter since the kneaded matter is sufficiently melted, and that the kneaded matter is extruded from an outlet with a certain degree of stress being applied thereto.
Moreover, Japanese Patent Kokai Publication No. 161153/1994 (H6-161153) has disclosed that temperature of a kneading process is set within 20 K based on melting temperature of a resin and output temperature of the resin is not more than 35 K from melt temperature of the resin. Thus, this application suggests that wax is evenly dispersed with a small particle size so that the filming and the subsequent black spots and the fog are prevented.
Furthermore, Japanese Patent Kokai Publication No. 266159/1994 (H6-266159) has disclosed that barrel temperatures at a front step and a rear step of a kneader, softening point of a toner, and output temperature are set so as to maintain a certain relationship. This application suggests that this arrangement makes it possible to further improve dispersion of an additive agent in a binder resin, to provide a uniform state, and also to improve charging property.
However, in recent demands for high picture quality and for recycling a waste toner, higher dispersing property for achieving highly uniform dispersion is required. Moreover, in color images in which high light-transmittance and anti-offset property need to be satisfied without using any oil, while a binder resin having low softening property with sharp melting characteristic should be used, dye and a charge control agent have to be finely dispersed in the binder resin; however, since a binder resin of low softening property is used, the above-mentioned twine-screw extruder fails to apply a sufficient shearing force, resulting in limited improvement of dispersing property of the dye, etc. In contrast, in the case when a binder resin having high softening property that has made to high molecular weight is used, light-transmittance of image decreases, and color reproducibility becomes poor in picture quality due to the high molecular weight component.
Moreover, in a mono-component developing system of a contact type which uses a developing roller made from a silicone resin, etc., and an elastic blade for regulating a toner layer and is provided with a supply roller for supplying toner to a developing roller, made from an urethane resin, etc., aggregation tends to occur in many places due to melt-adhesion to the blade and due to friction between a supply roller and a developing roller, resulting in poor image quality.
Moreover, as described above, from the viewpoint of recent earth environmental protection, it is preferable to recycle a waste toner that was left on a photosensitive member after a transferring process and has been collected by cleaning means, and again to use in a developing process. However, upon recycling a waste toner, the toner has been damaged due to stress, etc., applied thereto in a cleaner section, a developing section or a transporting tube through which the waste toner is returned to a developing unit.
Moreover, in the case when a waste toner that has been scraped from a photosensitive member during a cleaning process is again recycled in a developing process, if an internal additive agent and colorant are insufficiently dispersed, those particles insufficient in dispersion tend to form a waste toner, and when those particles are mixed with a new toner in a developing device, distribution of charge quantity becomes uneven, as a result, toner particles having reversed polarity increases and copied images becomes poor in quality.
Furthermore, in the case of a toner to which a low melting point component, such as wax, has been added, filming of the wax to a photosensitive member is promoted, resulting in a shorter service life. Here, in the case of a sheet of short paper, such as post cards, the paper is transported by frictional force up to a photosensitive drum, however a photosensitive member having filming is poor in transporting force, resulting in transportation failure of such a sheet of paper.
In the aforementioned transferring system using a conductive elastic roller, transfer paper is allowed to pass between an image bearing member and the conductive elastic roller, and by applying transfer bias voltage to the conductive elastic roller, toner on a surface of the image bearing member is transferred onto the transfer paper; however, the transferring system using the conductive elastic roller of this type has a problem in which the transfer paper is susceptible to stain on a rear face. The reason for this is explained as follows: In the case when a transferring process is carried out by a transferring toner on a image bearing member to transfer paper by using a transfer roller, the transfer roller is made in contact with the image bearing member with predetermined pressure when no transfer paper is applied, and when there is much fog during a developing process, the fog contaminates the transfer roller, and the transfer roller contaminated by the toner comes into contact with the rear face of transfer paper sent thereto. In toner particles in which the internal additive agent is insufficiently dispersed, there is reduction in fluidity, and toner aggregates partially; thus, a void image tends to appear during a transferring process. These phenomena become more frequently when waste toner is recycled.
An intermediate transfer system does not need any complex optical system, and is applied to sheets of paper that is not so flexible, such as post cards and card board, and it also provides flexible structure when the intermediate transfer belt is used; therefore, in comparison with a transfer drum system and a continuous transfer system, the system is more advantageous in that an apparatus may be miniaturized.
It is ideal that all the toner be transferred during a transferring process; however, toner partially remains after a transferring process. That is, so-called transferring efficiency is not 100%, and in general, it is approximately of 75 to 90%. A residual toner after a transferring process is collected by a cleaning blade, etc., in a photosensitive member cleaning process to form a waste toner.
However, in structure using an intermediate transfer member, a toner is subjected to at least two transferring processes, that is, the transferring processes from a photosensitive member to the intermediate transfer member and that from the intermediate transfer member to a sheet of image receiving paper; therefore, even when the transferring efficiency is, for example, 85% in a normal copying machine having one transferring process, the transferring efficiency is reduced to 72% after two times of the transferring processes. Moreover, in the case of the transferring efficiency of 75% in one transferring process, this is reduced to 56%, in which approximately half a toner becomes a waste toner; this results in high costs of a toner, and larger capacity of a waste toner box impede to miniaturize the apparatus. It is considered that the reduction in transferring efficiency is caused by fogging resulting from reversed polarity and void image during a transferring process, due to insufficient dispersion.
Moreover, in the case of a color developing process, a toner layer becomes thicker because toner images of four colors are superimposed on an intermediate transfer member; thus, pressure variation tends to occur between thicker toner portions and thinner toner portions or no toner portions. For this reason, so-called a “void” phenomenon, in which one portion of an image is not transferred due to aggregation effect of toner to form a hole, tends to occur. Moreover, in the case when a material having high toner mold-releasing effect is used as an intermediate transfer member so as to ensure a cleaning process in the event of an image-receiving sheet jam, the void phenomenon occurs more frequently, resulting in serious degradation in image quality. Furthermore, characters, lines, etc., are subjected to the edge developing process to have more toner, with the result that aggregation between toner particles occurs due to pressure application, making the void phenomenon more conspicuous. In particular, this becomes more conspicuous in high-temperature and high-humidity environments.
Moreover, in an electrophotographic apparatus which will be described later, a group of image-forming units in which a plurality of movable image-forming units, which form toner images of different colors, are arranged in ring shape are provided, and the entire image-forming units are allowed to rotate. Here, in the respective image-forming units and intermediate transferring units, those units are exchangeable so that maintenance processes are easily carried out by exchanging the units when an occasion for exchange is due after service life; thus, it is possible to provide an easy maintenance process in the same manner as a monochrome printing process even in the case of an electrophotographic color printer. However, since the image-forming unit itself is revolved, waste toner after having been cleaned temporarily adheres to a photosensitive member repeatedly, and since it repeats adhesion and separation to and from a developing roller, a photosensitive member is susceptible to damage and filming, and in the case of poor rising property of charge during an initial stage of the developing process, background fogging tends to occur.
Moreover, in a fixing process of the four-color toner image, it is necessary to mix color toners. In this case, when the toners are insufficiently melted, light scattering occurs on a surface of the toner image or inside thereof, with the result that color tone of inherent toner pigment is impaired and light is not made incident on a lower layer at overlapped portions, causing degradation in color reproducibility. Therefore, a toner needs to have complete melting property and also to have light-transmittance so as not to impair color tone, as essential requirements. In particular, along with increase of opportunities in which presentations are made by using color images through OHP, transparency of color images becomes more important.
However, in the above-mentioned resin composition, when an attempt is made to improve melting property, anti-offset property becomes poor, causing toner to adhere to a surface of a fixing roller without being all fixed on a sheet of paper, and resulting in offset; therefore, a great amount of oil, etc., needs to be applied onto the fixing roller, resulting in complex handling processes and device structures. Here, another method in which anti-offset property is improved by applying a mold-releasing agent such as polypropylene and polyethylene may be proposed; however, a great amount of addition thereof is required, causing reduction in dispersing property in the above-mentioned binding resin having sharp melting property and resulting in unclearness in color and subsequent degradation in color reproducibility.
Here, in Japanese Patent Kokai Publication No. 119509/1993 (H5-119509) and Japanese Patent Kokai Publication No. 220808/1996 (H8-220808) have disclosed that a great amount of addition of carnauba wax makes it possible to reduce color unclearness and to provide a superior fixing property and anti-offset property.
However, as described above, simple addition of carnauba wax still tends to cause background fogging, filming to a photosensitive member, a developing roller and an intermediate transfer member, an insufficient transferring process, and these phenomena becomes more conspicuous in a recycling process of a waste toner.
Here, toners need to generally satisfy the above-mentioned subjects.